Light intensity calorimeter

ABSTRACT

A calorimeter for measuring high-energy pulsed laser light. The light is absorbed by a suitable liquid which causes a molecular expansion. The liquid may be permitted to expand and the change of volume may be measured by a capillary tube. Alternatively, if the liquid is not permitted to expand, the rising pressure may be measured. This obviates the usual waiting period needed for a temperature equilibrium to be reached in conventional calorimeters.

United States Patent Briones [4 1 June 20, 197 2 [54] LIIGHT INTENSITYCALORIMETER OTHER PUBLICATIONS 2] Inventor! Robert Bfiones, GranadaHills, Calif- Wiloughby, A. B. Absolute Water Flow Calorimeter for theMeasurement of Intense Beams of Radiant Energy" In Rev. of [73]Ass'gnee' TRW Beach Scientific Instruments v01. 25 No. 7 July 1954 Pg.667- 669 Filed! April 1971 Damon et al. A Liquid Calorimeter for HighEnergy Lasers." [211 No: 139,268 In Applied Optics Vol. 2 No. 2 Feb.1963 pg. 163- 164 Related [1.8. Application Data PrimaryExaminer--Richard C. Queisser Assistant Examiner-Herbert Goldstein [63]Continuation of Ser. No. 592,381, Nov. 7, 1966, aban- A no m ey Dani dAnderson Gerald Singer and Alfons doned.

Valukoms [52] US. Cl. 73/190R 51 1m. (:1. ..(;011 17/00, G01t 1/16 [57]ABSIRACT [58] Field Of Search ..73/ 190; 250/833 A calorimeter formeasuring high-energy pulsed laser light, The light is absorbed by asuitable liquid which causes a [56] References cued molecular expansion.The liquid may be permitted to expand and the change of volume may bemeasured by a capillary UNITED STATES PATENTS tube. Alternatively, ifthe liquid is not permitted to expand, 1,089,743 3/1914 Brown ..73/355the rising pressure may be measured, This obviates the usual 1,940,75912/1933 Llncolnm "324/92 waiting period needed for a temperatureequilibrium to be 1 980,213 1 "1934 Lindsay reached in conventionalCalorimeters, 2,846,647 8/1958 MacPherson.. ...324/95 2,866,950 12/1958Smits ..324/95 4 Claims, 2 Drawing Figures pressure ou i put lightradiation PMENTEDmzo m2 3. 670,570

Loser Robert A. Briones,

INVENTOR.

AGENT.

LIGI'IT INTENSITY CALORIMETER This Application is a StreamlinedContinuation of application, Ser. No. 592,381, filed Nov. 7, 1966, nowabandoned.

This invention relates in general to energy measuring apparatus and moreparticularly to apparatus for measuring the amount of light emitted froma high-energy source such as a laser.

With the advent of lasers, there exists a need for diagnostic energymeasuring devices which are simple in function, cheap to produce and areeasily calibrated.

Existing devices are calibrated at low energies and then areextrapolated to the higher energy levels of a laser.

A prior art device for measuring energy by calorimeter technique isdescribed in US. Pat. No. 3,143,703, entitled Microwave CalorimeterWattmeter With A Reflectionless R. F. Termination", by L. D. Kraeuter.In the device of that patent, microwave energy is dispersed into a knownquantity of water. Because the water is a lossy conductor, rapiddissipation of the microwave energy takes place. The conversion ofmicrowave energy into heat energy raises the temperature of the water.The temperature of the water is then proportional to the microwaveenergy dissipated.

1n the apparatus of this invention, energy from a light source, such asa laser, is dissipated in a volume of light-absorbing liquid, causing achange in the temperature, which in turn causes a change in the volumeor the pressure of the liquid. In one embodiment a capillary columnprovides a path for the expanded liquid. The distance that the liquidrises in the capillary column is proportional to the light energyabsorbed by the liquid. In another embodiment the volume is heldsubstantially constant, causing the pressure to vary as a function ofthe absorbed light energy.

It is therefore an object of the present invention to provide a novelapparatus for measuring the energy of a light source and, in particular,a high energy pulsed laser light source such as a ruby laser or aneodymium doped glass laser.

It is another object of the present invention to provide an improvedhigh energy measuring apparatus utilizing the thermal expansionqualities of a liquid.

It is another object of the present invention to provide an apparatusfor measuring relatively high levels of light energies which also may becalibrated at these high energy levels.

These and other objects of the present invention will become moreapparent when taken in conjunction with the following description anddrawings wherein:

P16. 1 illustrates a first embodiment of the invention; and

FIG. 2 illustrates a second embodiment of the invention.

Referring to FIG. 1, the apparatus is shown comprised of a hollowspheroid body vessel 18 having a hollow etched capillary member 12projection therefrom. The opening in the capillary tube has a diameterd. An opening 14 is provided in the body member for the insertion of apredetermined amount of light absorbing fluid 19. This opening is thensealed by standard means well known to those persons skilled in the art.A heating element 15, such as a resistor, is inserted into the liquid 19and is electrically connected in circuit through the switch 17 to apower supply 16. The power supply may consist of a charged condenserwhich will deliver a known pulse of energy to the heating element. Theheat generated by element 15 is used to calibrate the expansion ofliquid 15. To calibrate the device, switch 17 is closed imparting aknown amount of energy into the liquid 19. The expansion of the liquidwill be proportional to the known amount of energy introduced. A window11 projects from the spherical body member 18 to provide a flathomogenous surface 11 through which the radiation from the laser lightsource 20 may impinge on and pass into the liquid 19. The outside of thesphere, except for the flat surface 11, may be aluminized or silvered tominimize any radiation loss, and to reflect the radiation back into thevessel.

The apparatus operates on the principle of thermal expansion of liquids.namely,

where V, is the initial volume of fluid, AT is the change in temperatureand a, B, and 'y are the three volumetric expansion coefficients.

For small changes in temperature, one is justified in using the linearapproximation of the above:

AV E V, aAT (2) The change in temperature is determined by the specificheat c,,", mass of the absorbing liquid pV," and the amount of heatabsorbed Q":

AQIPVICP) (3) Substitution of Eq. (3) in (2) shows: AV=(AQa/pc (4) Fromthese equations, it can be seen that the change in volume is independentof the volume of liquid employed. Therefore, I propose that a sphere, orany other shape filled with a light absorbing liquid, can be used tomeasure the amount of energy emitted by a source such as a laser orflash lamp. Since the absorption of the energy is local, the change involume is instantaneous; thus, no thermal gradient or time constant ispresent to introduce error in peak readings due to heat loss. One onlyneeds to insure that all of the energy is absorbed. This may requiredoping the liquid with an appropriate dye.

At present, there exist several commercial laser calorimeters which arebased upon the measurement of the temperature change of a liquid viathermal electric elements imbedded in the fluid column. (Laser Fare,Vol. 1, No. 1, published by Korad Corporation, Jan. 1965.) These systemsrequire detailed knowledge of the heat capacities of the variouscomponent parts. Accurate measurement requires involved analysis of heatlosses and thermal time constants. In brief, calibration is indeedindirect and difiicult. The output of these systems cannot .be readwithout the aid of a potentiometer or microvoltmeter.

The apparatus of this invention completely eliminates calibrationdifficulties, requires no external electronics or measuring circuitsoutside of an engraved scale. This type of light intensity calorimeteris not dependent upon thermal equalization or upon thermal gradients.For the embodiment shown in the drawing =1? (pe /a) Ah (5) A list ofliquids which nt ay be used on'the light absorbing liquid 19 is given inthe following chart:

From the above chart, it can be seen that there is a large range ofliquids that can be used, depending upon the energy level to be measuredand the available size capillary. Since these particular liquids aretransparent, an appropriate dye can be used, such as an analine dye, tomake the absorption of the input energy efiicient.

In conclusion, the important feature of this invention is that it doesnot matter how the optical energy is distributed in the sphere. Thechange in volume produced is independent of the distribution of the heatin the sphere and depends only on the total heat absorbed by the liquid.It is, therefore, unnecessary to wait to take measures to insure auniform temperature distribution as in the case of a thermocouple typecalorimeter.

Referring to FIG. 2, the apparatus 10 is showncomprised of a hollowspheroid body vessel 24. The vessel 24 is made of a material which willnot flex any substantial degree under the influence of pressure.

A window 21 which is transparent to light is inserted through the wallof the vessel to provide a passageway for light radiation. A lightabsorbing liquid 23 fills the vessel. A pressure transducer 26 projectsthrough the wall of vessel 24 to measure the pressure exerted by theliquid against the wall of 5 the vessel. A heating element is immersedin the liquid 23. Power supply 16 is connected to the heating element bymeans of switch 17. The leads from the heating element to the switch andpower supply are insulated from the walls of the vessel by means ofinsulators 25. The heating element provides a known amount of thermalenergy to the liquid so that the instrument may be calibrated. A reliefvalve 22 is inserted through the wall of the vessel to re-zero anyundesired pressure buildup.

While there has been shown what are considered to be the preferredembodiments of the invention, it will be manifest that many changes andmodifications may be made therein without departing from the essentialspirit of the invention. it is intended, therefore, in the annexedclaims to cover all such changes and modifications as fall within thetrue spirit of the invention.

WHAT IS CLAIMED IS:

1. A radiant energy calorimeter comprising:

a. a closed container containing a radiant energy absorbing liquidhaving a substantial molecular expansion upon ab sorption of heat due toradiant energy;

b. a source of radiant energy impinging on the liquid in said containerfor imparting pulsed radiant energy thereto. thereby to increase thepressure of the liquid; and

c. a pressure transducer communicating with the interior of saidcontainer for measuring the presure increase caused by the molecularexpansion imparted to the liquid by the pulsed energy of said radiantsource.

2. A light energy calorimeter as defined in claim 1 which furthercomprises a dye mixed with said liquid to increase the light absorptionthereof.

3. A light energy calorimeter as defined in claim 1 wherein means areprovided for imparting a known amount of energy to said liquid andincluding a heating element disposed in said container and means forsupplying thereto a pulse of known electric energy so as to calibratesaid calorimeter.

4. A light calorimeter as defined in claim 1 wherein said vessel iscoated at least partially with a light reflecting material, thereby tominimize reradiation of the light energy absorbed by said liquid.

i t i i

1. A radiant energy calorimeter comprising: a. a closed containercontaining a radiant energy absorbing liquid having a substantialmolecular expansion upon absorption of heat due to radiant energy; b. asource of radiant energy impinging on the liquid in said container forimparting pulsed radiant energy thereto, thereby to increase thepressure of the liquid; and c. a pressure transducer communicating withthe interior of said container for measuring the pressure increasecaused by the molecular expansion imparted to the liquid by the pulsedenergy of said rAdiant source.
 2. A light energy calorimeter as definedin claim 1 which further comprises a dye mixed with said liquid toincrease the light absorption thereof.
 3. A light energy calorimeter asdefined in claim 1 wherein means are provided for imparting a knownamount of energy to said liquid and including a heating element disposedin said container and means for supplying thereto a pulse of knownelectric energy so as to calibrate said calorimeter.
 4. A lightcalorimeter as defined in claim 1 wherein said vessel is coated at leastpartially with a light reflecting material, thereby to minimizereradiation of the light energy absorbed by said liquid.